The reaction or complexation of a drug with an ion exchange resin forms a composition known as a drug-resin complex. A drug for the purposes of the present invention is a medicinal substance for internal or external use. An ion exchange resin is an ionic, or charged, compound which has binding sites that can bind or take up an ionic drug. The most common types of ion exchange resins are polymers. Such a compound is called a resin because the polymer is formed into very small particles or beads.
Drug-resin complexes have several advantages over pure drugs in ordinary formulations. Many drugs are bitter and some smell bad. Getting a patient, particularly a small child or an elderly person, to swallow something that tastes or smells bad can be a serious problem. Complexing such a drug with a resin often improves the taste or the smell.
Complexing a drug with a resin can also change its physical characteristics. This change may make the drug more convenient to mass produce or easier for patients to take. For example, mixing a drug in powder form with inert ingredients and compressing the mixture into a tablet is a very common and inexpensive way of preparing a drug for consumption. However, if a particular drug in liquid or powder form tends to make a crumbly or sticky mixture, large-scale automated tablet compression may be impossible or overly costly. Complexing a drug with a resin can sometimes improve compression characteristics.
Complexing a drug with a resin can affect the rate at which the drug dissolves in the digestive system of a patient. Fast dissolution can be a problem if it means the drug has to be taken often to maintain a reasonably even level of the drug in the blood. If a drug causes stomach upset when it dissolves, rapid dissolution in the stomach may also be undesirable. Drug-resin complexes often dissolve more slowly than an ordinary drug formulation. Complexes are useful in changing dissolution profiles and are frequently used in time-release formulations. Coating of a drug-resin complex can delay the release of a drug even more.
The technique for adsorption of a drug onto an ion exchange resin to form a drug-resin complex is well-known. Generally the drug is mixed with an aqueous suspension of the ion exchange resin and the complex is dried. Complexation of the drug by the resin may be detected by a change in pH or by other changes in physical properties or by a decrease in concentration of drug dissolved in the aqueous phase.
Ion exchange resins are usually made from a polymer backbone with various displaceable functional groups ionically bonded to the polymer. In water the functional groups of the resin ionize. The polymer chains are also typically cross linked, leading to a gel-like insoluble composition formed in beads. The particle size of a resin can differ between two resins even though the polymer it is made from is the same. The amount of cross linking also varies from one resin to another. The amount of drug which can be bound to a particular resin is called its binding capacity or loading. Binding capacity varies greatly between resins and from drug to drug. Most resins are sold in dehydrated form and then soaked in water prior to use.
Cationic ion exchange resins have negatively charged, or anionic, binding sites. The anionic binding sites are bonded to displaceable cationic groups. Cationic drugs are positively charged and tend to displace the cationic groups, typically becoming bonded to the resin by ionic bonds. Since basic drugs are generally cationic, cationic exchange resins are often used to prepare drug-resin complexes with basic drugs. Typical approaches to forming a water insoluble drug-resin complex are to react the sodium salt of a cationic ion exchange resin with a cationic drug or to react the base form of the drug with the acid form of the cationic ion exchange resume.
Anionic ion exchange resins have positively charged, or cationic, binding sites. The cationic binding sites are bonded to displaceable anionic groups. Anionic drugs are negatively charged and tend to displace the anionic groups, typically becoming bonded to the resin by ionic bonds. Since acidic drugs are generally anionic, anionic exchange resins are frequently used to prepare drug-resin complexes for acidic drugs. Once a drug-resin complex reaches the digestive system of a patient, the many ions present there tend in turn to displace the drug from the resin and release the drug.
Many drugs have been found to be chemically unstable when reacted with a resin. The drug alone does not degrade in the same way. The decomposition products generally are oxidized forms of the drug, or in some cases hydrolytic products. This decomposition occurs both in the presence of water and when the drug-resin complex is dry. U.S. Pat. No. 5,413,782 (Warchol et al.) describes a method for increasing take-up of the drug and preventing decomposition of anionic drug-ion exchange resin systems. This method involves, not adding a chemical, but rather reacting the drug and the resin in the absence of carbon dioxide and/or bicarbonate ion.
The use of chelating agents to stabilize chemicals and drugs in solution is known. Chelating agents are scavengers for trace amounts of metal ions. Chelation refers to the formation of an unusually stable bond between an organic compound and an ion or other polar group. Most commonly chelation involves a metal ion. The unusual stability of the bond is due to the ability of the organic compound to bind to a central ion at two or more binding sites, often in a ring formation. Compounds which have this ability are known as chelating agents or chelating ligands. The resulting combination of a chelating ligand with a metal ion is referred to as a metal complex. Many reactions, including many oxidation and decomposition reactions, are catalyzed by trace amounts of metallic ions present in solutions. Many drugs can be degraded through oxidation and hydrolytic reactions which are catalyzed by metal ions. The presence of metallic ions can therefore significantly accelerate the degradation of these drugs. Chelating agents are useful in preventing degradation for drugs in solution. EDTA (ethylene diamine tetraacetic acid) and its salts are examples of powerful chelating agents. EDTA is known to stabilize drugs in solution by retarding their oxidation.
U.S. Pat. No. 4,973,607 (Stahlbush et al.) describes the use of antioxidants to improve the chemical stability of cationic exchange resins. This differs from the present invention in that only the resin is involved, not a drug-resin complex. U.S. Pat. No. 4,221,778 (Raghunathan) describes prolonged release pharmaceutical preparations made of ion exchange resin drug complexes treated with a solvating agent and provided with a diffusion barrier coating.
U.S. Pat. No. 5,368,852 (Umemoto et al.) describes prolonged release liquid pharmaceutical preparations of drug-resin complexes coated with ethylcellulose and including a benzoate preservative to reduce bacterial activity. U.S. Pat. Nos. 5,182,102 (DeSantis, Jr. et al.) and 5,540,918 (Castillo et al.) describe drug-resin ophthalmic compositions whose resistance to bacterial contamination is improved by the use of antimicrobials. EDTA is disclosed as an antimicrobial in such compositions.
U.S. Pat. No. 4,894,239 (Nonomura et al.) discloses preparations that contain drug-resin complexes in which an antioxidant may be added. U.S. Pat. No. 5,152,986 (Lange et al.) also discloses preparations that contain drug-resin complexes in which an antioxidant may be added.
U.S. Pat. No. 4,448,774 (Clemente et al.) discloses aqueous pharmaceutical solutions that contain a drug, a pharmaceutically acceptable preservative such as sodium benzoate, and a chelating agent such as ethylene diamine tetraacetic acid. None of the patents described above discloses a pharmaceutical composition in the form of a solid or gel that comprises a drug-resin complex and a chelating agent.